Local Area Networks (LANs) connect computing systems together at the Layer 2 level. The term “Layer 2” refers to the second layer in the protocol stack defined by the well-known Open Systems Interface (OSI) model, also known as the logical link, data link, or Media Access Control (MAC) layer. Each computing system connects to a LAN through a MAC device. Multiple LANs can be connected together using MAC bridges, as set forth in the IEEE Standard for Information Technology, Telecommunications and Information Exchange between Systems, Local and Metropolitan Area Networks, Common Specifications, Part 3: Media Access Control (MAC) Bridges, published as ANSI/IEEE Standard 802.1D (2004), which is incorporated herein by reference. (The 802.1D standard, as well as other IEEE standards cited herein, is available at standards.ieee.org/catalog/.) MAC bridges that implement the 802.1D standard allow MAC devices attached to physically separated LANs to appear to each other as if they were attached to a single LAN. The bridge includes two or more MAC devices that interconnect the bridge ports to respective LANs.
MAC bridges maintain a forwarding database (FDB) to map destination MAC addresses of the packets they receive to bridge ports. The bridge builds the forwarding database by means of a learning process, in which it associates the source MAC address of each incoming packet with the port on which the packet was received. When the bridge receives an incoming packet whose destination address is not found in the database, it floods (i.e., broadcasts) the packet through all its available ports, except the one through which the packet arrived. Other MAC bridges that do not recognize the destination address will further flood the packet to all the relevant ports. Through the flooding mechanism, the packet will eventually traverse all interconnected bridges at least once, and will ultimately reach its destination.
Recently, various means have been proposed and developed for transporting Layer-2 packets, such as Ethernet frames, over high-speed, high-performance Layer-3 packet networks. Methods for this purpose are described, for example, by Martini et al., in “Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks” (IETF draft-ietf-pwe3-ethernet-encap-11.txt, November, 2005), which is incorporated herein by reference. This draft, as well as other Internet drafts cited herein, is available from the Internet Engineering Task Force (IETF) at www.ietf.org/internet-drafts. The draft defines mechanisms for encapsulating Ethernet traffic for transportation over Internet Protocol (IP) networks using Multi-Protocol Label Switching (MPLS) or other tunneling methods, such as Generic Routing Encapsulation (GRE), as are known in the art.
According to the model proposed by Martini et al., native Ethernet LANs are connected to the IP network by provider edge (PE) devices, which are linked one to another by tunnels through the IP network. As a result of the encapsulation of Ethernet frames and associated processing functions, the IP network emulates Ethernet trunking and switching behavior and can thus be treated as an Ethernet “Pseudo-Wire” (PW). In other words, from the point of view of native Ethernet LANs that are connected to tunnels through the IP network, each PW is a virtual Ethernet point-to-point connection, emulating a physical connection between two Ethernet ports. Martini's encapsulation method may also be used in conjunction with virtual LANs (VLANs), as defined in IEEE standard 802.1Q.
Taking this functionality a step further, a number of authors have described methods for creating a virtual private LAN service (VPLS), which links different LANs together over an IP network. Such methods are described, for example, by Kompella et al., in “Virtual Private LAN Service” (IETF draft-ietf-12vpn-vpls-bgp-06.txt, December, 2005) and by Lasserre et al., in “Virtual Private LAN Services over MPLS” (IETF draft-ietf-12vpn-vpls-1dp-08.txt, November, 2005), which are incorporated herein by reference.
A VPLS (also known as a transparent LAN service—TLS) provides bridge-like functionality between multiple sites over a large network. Users connect to the VPLS via regular Ethernet interfaces. PWs between the nodes to which the users are connected form the VPLS entity itself. Every node in a VPLS acts as a virtual bridge. A virtual bridge node has “virtual ports,” which are the endpoints of PWs that are part of the VPLS. The interfaces to which the users are actually connected are physical ports at the network edges. Both virtual and physical interfaces are treated identically from the point of view of frame forwarding and address learning. A single provider node can participate in multiple VPLS instances, each belonging to a different user. From the perspective of the end-user, the VPLS network is transparent. The user is provided with the illusion that the provider network is a single LAN domain. User nodes on different physical LANs can thus be joined together through VPLS connections to define a Layer 2 virtual private network (VPN), which appears to the users to be a single Ethernet LAN.
Link aggregation (LAG) is a technique by which a group of parallel physical links between two endpoints in a data network can be joined together into a single logical link (referred to as the “LAG group”). Traffic transmitted between the endpoints is distributed among the physical links in a manner that is transparent to the clients that send and receive the traffic. For Ethernet networks, link aggregation is defined by Clause 43 of IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications (2002 Edition), which is incorporated herein by reference. Clause 43 defines a link aggregation protocol sub-layer, which interfaces between the standard Media Access Control (MAC) layer functions of the physical links in a link aggregation group and the MAC clients that transmit and receive traffic over the aggregated links. The link aggregation sub-layer comprises a distributor function, which distributes data frames submitted by MAC clients among the physical links in the group, and a collector function, which receives frames over the aggregated links and passes them to the appropriate MAC clients.